Physical Nature of Neutral Instability

Experience has established the fact that the neutral of a three-phase system may become subject to certain strange phenomena of instability under apparently normal conditions. The disturbances are of two distinct classes: (1) persistent shift or inversion of the neutral, resulting in unequal leg voltages; and, (2) persistent oscillation of the neutral (with equal effective voltage in all three legs) at approximately one-half, double, or triple frequency. While in the ultimate analysis saturation is at the basis of the phenomena, a more definite explanation is given as follows: 1. Neutral shift or inversion is a fundamental frequency phenomenon, and is due to the fact that the volt-ampere curve of the combination of an iron core reactor (transformer magnetizing current) in shunt with a suitable capacitor has one zone which is lagging and one which is leading. In a Y-Y bank of transformers, with suitable balanced line capacitances to neutral following a switching disturbance, one leg may act leading, the others lagging, and thus invert the neutral. 2. Oscillations of the neutral´tend to take place at its natural frequency, but since, due to inevitable losses, not all oscillations can persist, in course of the starting transient the oscillation is resolved to the nearest lower frequency which is able to draw energy from the circuit by approximating harmonic relationship to it. The even harmonics are accounted for by the persistence of residual in the core, whether left from previous excitation or brought about by the direct current component of starting transient.